The present invention relates, in general, to an electromagnetic switchgear drive, and more particularly to a magnetic drive mechanism incorporating a yoke which encloses a permanent magnetic arrangement and which surrounds a coil form. The coil form has a continuous bore which contains a movable armature that is connected by means of a reverse-transfer lever to actuate a movable slider mounted with allowance for sliding in a contact apparatus. The slider is located above the coil form, parallel to the longitudinal axis thereof, and is in operative contact with two return springs preloaded in opposite directions, which urge the slider to a neutral, or central, position. The slider carries spring-loaded bridge contacts to each of which are assigned two opposed fixed contacts of the switchgear.
The type of switchgear to which the present invention relates is described in applicants' German patent application No. P 31 38 265.7 which is acknowledged as disclosing the state-of-the-art, but which has not been published as prior art. The armature in that prior device is mounted with allowance for tilting in the vicinity of one of two coil form flanges, and extends with both ends beyond the flanges. The armature cooperates with pole faces on the yoke and with a permanent magnet arrangement in which two rod-shaped permanent magnets lie above the coil form and are parallel thereto.
In applicants' prior patent application, a leaf spring is provided which engages a reverse-transfer lever, with the opposite end of the spring leaf bearing against the housing of the contact apparatus. In the case of a mono- or bistable design of the switchgear, the spring is made of one piece, but in the case of a tristable design; i.e., with a central position of the adjustable slide in which all contacts are open, the spring consists of two spring leaves which are pre-loaded in opposite directions. Whereas the mono- and bistable operations of that switchgear offers no special features; i.e., the direction of motion of the armature and, thereby, of the adjusting slide is determined, as in the case of any polarized magnetic drive, by the direction of current flow through the coil, tristable operation presents a difficulty. To obtain tristable operation, a drop-out pulse with an accurately predetermined current level and duration is needed for reaching the central position in order, on the one hand, to achieve a secure drop-out and, on the other hand, to prevent overshooting of the central position. In that prior device, it is also of importance that the desired course of the return spring force be obtained as a function of the armature path by means of the split spring leaf described above.